The present invention relates to an optical system having a color-encoding filter means, for example in a single tube color television camera, and an optical low-pass filter in an objective lens system for attenuating beats appearing on the television picture, and more particularly, it relates to and contemplates an optical system in which the optical low-pass filter is formed of a rectangular wave phase grating including a plurality of sets of laminae for introducing phase retardation, the laminae of each set being arranged in parallel with each other and the laminae of different sets being arranged at an angle with each other.
In the prior art, for example, in television cameras of a single image tube or two image tube type, it has been well known to use a color-encoding filter means disposed in the light path of an optical system so as to obtain three primary color signals.
In such an optical system in a television camera, it has been also well known that a beat occurs on the picture image in the color television which is due to the fact that a circuit for detecting color signals in a color television system misdetects high frequency components with regard to or along the scanning direction of the image tube in a picture image on the color-encoding filter as chrominance signals.
In order to attenuate such a beat, it is required that high frequency components of the objective image on the color-encoding filter are attenuated below a predetermined level. In order to satisfy this demand, in the optical system there is provided an optical low-pass filter to effect a degree of blur larger than the value calculated from the grating period of the color-encoding filter to the objective image on the color-encoding filter, regardless of the f-number and the diaphragm aperture value of the objective lens system.
As such an optical low-pass filter, it has been known that a polyhedron prism, nylon mesh, or the like has been used, however, each of these is highly difficult to manufacture and also to incorporate into the photographic lens and is, accordingly, not suitable for practical use.
A unique low-pass filter composed of an asymmetrical rectangular wave phase grating, in which on a transparent base plate laminae of equal widths for giving a phase retardation are regularly disposed with identical pitches in an identical direction, has been disclosed in the U.S. patent Ser. No. 379,013 filed July 13, 1973, now U.S. Pat. No. 3,756,695, and having the same assignee as the present application. Said optical low-pass filter has the advantage of being simple to manufacture because it can be obtained through the conventional vacuum evaporation method and it is easy to incorporate it into the objective lens. While it has the drawbacks in that when a degree of blur in the image of the principal object to be focused on the color-encoding filter is beyond a predetermined level, the beats due to other kinds of high frequency components undesirably appear by inserting said low-pass filter.
The present invention aims to remove the above drawbacks and provide an optical low-pass filter in which all object images which are focused or not focused on the color-encoding filter cause no beats.